Diesel engine exhaust traps are currently being manufactured by Corning and others. The trap is of ceramic, and is placed in line in series with the exhaust pipe, so that all exhaust gases must go through the trap. The trap is constructed in such a way that the gases must literally pass through a ceramic wall with a very small pore size, in order to escape to the atmosphere. If the pores are too small, or become clogged, then a back pressure arises upstream of the trap, which can be sensed.
Diesel engines can operate under full load or under some fraction of full load. An engine operating at or close to full load will normally develop quite high exhaust temperatures, in the region of 500.degree. C. The exhaust gases contain unburned carbon and normally also contain unburned organics (hydrocarbons for example), which typically are in the vapourized state at that temperature.
The ceramic trap captures the carbon, because the carbon particles are too large to pass through the small pores of the walls. What is intended is that the carbon be ignited within the trap and burned to CO or CO.sub.2, thus disappearing from the trap with the resultant gases passing through the walls of the trap.
If a diesel engine is operating at a fraction of the full load, it will not develop high exhaust temperatures. The exhaust temperatures may be only around 200.degree. C., and this low temperature is generally insufficient to ignite the carbon within the trap. Conventionally, one approach to this problem is to lower the ignition temperature of the carbon in the trap, and one way of doing this is to coat the inside of the wall with a catalyst, for example a precious metal such as platinum or palladium. Another approach is to put a metal additive into the fuel, which may be organic manganese or copper manganese. The result is that the carbon particles intimately contain the manganese. Copper and manganese are low-activity catalysts.
The disadvantage of using a precious metal catalyst is that, because these metals are so active, they can also oxidize SO.sub.2 to H.sub.2 SO.sub.4.
A different approach is to coat the ceramic trap itself with a base metal catalyst such as manganese or copper. This requires a higher temperature to operate properly, but under some conditions is acceptable.
The foregoing is known technology, and the present applicant has been utilizing this technology in work with underground diesel installations, where the diesel engines typically operate at an 80% load factor. Thus, the engine typically runs at least 80% of full rated load, which is a very hot running condition. For this reason, the known technology works satisfactorily due to the high temperatures of the exhaust gases.
However, for trucks and other vehicles, particularly those running on city streets like buses, the diesel engine has a very light load duty cycle (from 20% to 30% typically), and the result is a very low exhaust gas temperature, typically about 200.degree. C., as mentioned earlier.
Currently, and especially in the United States, very stringent standards of particulates in emissions are being legislated. The requirements for 1990 are so rigorous that all such diesel engines will probably require traps.
Even though the catalytic technology described above improves the functioning of the trap at low duty cycles, nonetheless this technology does not function satisfactorily in all instances. Even the location of traps close to the engine in order to make use of maximum exhaust temperatures to burn off the accumulated particulate is of little use when the exhaust temperatures are low as a result of a low duty cycle. If combusting of the trapped particulate does not occur, then the trap will eventually become plugged with accumulated soot, thus interfering with proper operation of the diesel engine due to exhaust back-pressure.
More particularly, it is found that the use of a catalyst can reduce the ignition threshold temperature in the trap by 150.degree. C. to 350.degree. C., with the trap in the normal location in the exhaust system, i.e. close to the engine. An ignition threshold of 350.degree. C., however, still presents problems under some vehicle driving conditions, such as stop and go traffic, where exhaust temperatures are normally insufficient to reach the 350.degree. C. threshold.